Police believe a nerve agent was used to deliberately poison a former Russian double agent and his daughter, Britain's top counter-terrorism officer said on Wednesday, in a case that threatens to further damage London's ties with Moscow.

Sergei Skripal, once a colonel in Russia's GRU military intelligence service, and his 33-year-old daughter, Yulia, were found slumped unconscious on a bench outside a shopping centre in the southern English city of Salisbury on Sunday afternoon.

Both remain critically ill and a police officer who attended the scene is also in a serious condition in hospital.

The case is reminiscent of the 2006 murder of ex-KGB agent Alexander Litvinenko, who was killed with radioactive polonium-210 in London.

A previous British inquiry said Russian President Vladimir Putin probably approved the murder of Litvinenko, who died after drinking green tea laced with the rare and very potent radioactive isotope at London's Millennium Hotel.

"This is being treated as a major incident involving attempted murder by administration of a nerve agent," London Assistant Commissioner Mark Rowley told reporters of the Skripal case.

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"I can also confirm that we believe the two people originally who became unwell were targeted specifically."

Rowley declined to elaborate on the specific nerve agent, nor would he give specific details about how it was used to poison Skripal, 66, and his daughter.

England's chief medical officer said the incident posed a low risk to the wider public.

While Rowley would not say any more about the investigation, a U.S. security source, speaking on condition of anonymity, said the main line of police inquiry was that Russians may have used the substance against Skripal in revenge for his treachery.

Skripal betrayed dozens of Russian agents to British intelligence before his arrest by Russian authorities in 2004.

He was sentenced to 13 years in prison in 2006 after a secret trial and in 2010 was given refuge in Britain after being exchanged for Russian spies caught in the West as part of a Cold War-style spy swap at Vienna airport.

On Tuesday, British Foreign Secretary Boris Johnson said if Moscow were behind the incident then Britain could look again at sanctions and take other measures to punish Russia, which he cast as a "malign and disruptive" state.

Russia denied any involvement, scolded Johnson for "wild" comments and said anti-Russian hysteria was being whipped up intentionally to damage relations with London.

"It's very hard not to assess this (speculation) as provocative black PR designed to complicate relations between our two countries," Foreign Ministry spokeswoman Maria Zakharova told reporters in Moscow on Wednesday.

‘Need to deter Russia’

Police said new cordons had been added near Solstice Park, a business park, in the town of Amesbury near to Salisbury. They have sealed off the area of Salisbury where Skripal was found as well as the Zizzi pizza restaurant where they dined and the Bishop's Mill pub where they had a drink.

The British capital has been dubbed "Londongrad" due to the large quantities of Russian money which have poured in since the 1991 fall of the Soviet Union. It is the Western city of choice for many oligarchs from the former Soviet Union.

Russia denied involvement in the death of Litvinenko, which the British inquiry said had been hatched by the Federal Security Service (FSB), main successor to the Soviet-era KGB.

Former British defence minister Michael Fallon called for a stronger response if Russia was involved in the Skripal affair.

"We've got to respond more effectively than we did last time over Litvinenko. Our response then clearly wasn't strong enough," Fallon told Reuters. "We need to deter Russia from believing they can get away with attacks like this on our streets if it's proved."

Litvinenko's murder sent Britain's ties with Russia to what was then a post-Cold War low. Relations suffered further from Russia's annexation of Crimea and its military backing for Syrian President Bashar Assad.

The Skripal case has come shortly before Russia's presidential election on March 18, which Putin is expected to win comfortably, extending his rule by a further six years. The former KGB officer has been president since 2000.

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